Method and apparatus for subscriber line control circuit

ABSTRACT

A subscriber line interface circuit apparatus includes a linefeed circuit and a subscriber line control circuit (SLCC). In an embodiment, the linefeed circuit includes a signal conversion circuit which provides a differential mode signal and a common mode signal in response to at least the tip signal and the ring signal from the subscriber loop. The linefeed circuit includes a tip control circuit and a ring control circuit. In an embodiment, the SLCC is provided in a single integrated circuit chip and is coupled to the linefeed circuit which isolates the SLCC from the tip or ring signals. The SLCC includes a first and a second differential mode inputs for receiving the differential mode signal, and a common-mode input for receiving the common-mode signal. In an embodiment, the SLCC also provides various tip and ring control signals to the tip control circuit and the ring control circuit, respectively.

BACKGROUND OF THE INVENTION

The present invention is directed to integrated circuits. More particularly, the invention provides a method and device for an integrated circuit for telecommunication applications. Merely by way of example, the invention has been applied to subscriber line control circuits for interfacing with digital equipment in a central office. But it would be recognized that the invention has a much broader range of applicability. For example, embodiments of the invention can find application in communication networks such as cable television networks, fiber optic, Ethernet port interface to the Internet, VoIP, and wireless local loop, etc.

In a Plain Old Telephone Service (POTS) system, several functions are needed for telephony signaling, subscriber line supervision, and telephone equipment operations. These functions include battery feed, over voltage protection, ringing, supervision, codec, hybrid, and testing, etc. These electronic functions are collectively known by the acronym BORSCHT. BORSCHT circuitry is often found on a telecommunications network line card for implementation of a standard POTS telephone interface for digital equipment in a central office. With the advancement of integrated circuit technology, BORSCHT circuitry can be integrated into a chipset. For example, a line card may include a SLIC, or a subscriber line interface circuit, and a CODEC circuit. In a conventional design, the SLIC typically handles the analog functions and drives the telephone lines and usually include high voltage devices and circuits, whereas the CODEC typically handles digital signal processing and interface functions. Some conventional line cards may include a SLIC integrated circuit and a CODEC integrated circuit. A conventional line card may also include certain discrete components such as switching relays, large capacitors, etc. These conventional techniques have many limitations.

From the above, it is seen that an improved technique for subscriber line interface circuit is desired.

BRIEF SUMMARY OF THE INVENTION

According to an embodiment of the invention, in a subscriber line interface circuit apparatus for a subscriber loop, a linefeed circuit in combination with subscriber line control circuitry (SLCC) is provided wherein a first node is coupled to a first signal line and a second node is coupled to a second signal line to receive a tip signal and a ring signal, respectively, from the subscriber loop, with the improvement that the linefeed circuit includes a unique signal conversion circuit which is coupled to the first node and the second node to receive the tip signal and the ring signal, respectively and which provides both a differential mode signal and a related common mode signal in response to at least the tip signal and the ring signal so the SLCC circuitry need not derive such signals. In a specific embodiment, the common mode signal is associated with a sum of the tip signal and the ring signal, and the differential mode signal is associated with a difference between the same tip signal and ring signal.

In a specific embodiment, the SLCC includes a pair of voltage signal outputs for driving amplifiers associated with the tip and ring control circuits, in addition to an amplifier circuit for receiving the voice band or audio band signal from the linefeed circuit, an impedance matching circuit which is coupled to the amplifier circuit, a hybrid circuit coupled to the impedance matching circuit, and a codec which is coupled to the hybrid circuit. In a specific embodiment, the linefeed circuit includes discrete components, whereas in another embodiment, the linefeed circuit is implemented as an integrated circuit.

According to a specific embodiment, the present invention provides a subscriber line interface circuit apparatus for a subscriber loop. The apparatus includes a linefeed circuit and a subscriber line control circuit (SLCC). In an embodiment, the linefeed circuit includes a first node coupled to a first signal line and a second node coupled to a second signal line to receive a tip signal and a ring signal, respectively, from the subscriber loop. The linefeed circuit includes a signal conversion circuit which is coupled to the first node and the second node to receive the tip signal and the ring signal, respectively. The signal conversion circuit provides a differential mode signal and a common mode signal in response to at least the tip signal and the ring signal. In a specific embodiment, the common mode signal is associated with a sum of the tip signal and the ring signal, and the differential mode signal is associated with a difference between the tip signal and the ring signal. The linefeed circuit includes a filter circuit which is coupled to the subscriber loop and provides a voice band or audio band signal to the SLCC. In some embodiments, audio band includes a frequency range of about 20 Hz to 20,000 Hz, and voice band has a frequency range of about 300 Hz to 3,400 Hz. The linefeed circuit also includes a tip control circuit and a ring control circuit. The tip control circuit is coupled to the first node and provides a tip output signal to the first signal line in response to a first and a second tip control current signals and a tip control voltage signal. The ring control circuit is coupled to the second node and provides a ring output signal to the second signal line in response to a first and a second ring control current signals and a ring control voltage signal.

In a specific embodiment, a subscriber line control circuit (SLCC) is provided in a single integrated circuit chip. In an embodiment, the SLCC is coupled to the linefeed circuit and is isolated from the first and second signal lines. In an embodiment, the linefeed circuit isolates the SLCC from the first and second signal lines. In other words, SLCC does not sense the tip signal or the ring signal according to embodiments of the present invention. In contrast, the SLCC receives a differential mode signal and a common mode signal, and can be relatively more isolated from the tip and ring signals. In a specific embodiment, the SLCC includes a first and a second differential mode inputs and a common mode input. The differential mode inputs receive the differential mode signal from the linefeed circuit. The common mode input receives the common mode signal from the linefeed circuit.

In an embodiment, the SLCC also includes an analog to digital converter circuit which is coupled to the common mode input and the differential mode inputs. The SLCC also includes an analog control circuit which is coupled to the common mode input and the differential mode inputs and provides the individual tip and ring control signals in response to at least the common mode signal and the differential mode signal, without providing DC common mode control of the Tip and Ring signals. These control signals are described below. The SLCC includes a first and a second tip control outputs coupled to the analog control circuit and provides the first and the second tip control current signals, respectively. The SLCC includes a first and a second ring control outputs coupled to the analog control circuit and provides the first and the second ring control current signals, respectively. The SLCC also includes a tip control voltage output coupled to the analog control circuit, which provides the tip control voltage signal to the linefeed circuit, and a ring control voltage output coupled to the analog control circuit, which provides the ring control voltage signal to the linefeed circuit. The SLCC includes an amplifier circuit for receiving the voice or audio band signal. The SLCC also includes an impedance matching circuit which is coupled to the amplifier, a hybrid circuit coupled to the impedance matching circuit, and a codec which is coupled to the hybrid circuit.

In a specific embodiment, the subscriber line interface circuit apparatus described above includes a signal conversion circuit that includes first and second transistors. A first terminal of the first transistor is coupled to a resistor connected to the tip signal, a second terminal of the first transistor is coupled to a resistor connected to the ring signal, a third terminal of the first transistor is coupled to a first differential mode signal line, a first terminal of the second transistor is coupled to a resistor connected to the ring signal, a second terminal second transistor is coupled to a resistor connected to the tip signal, and a third terminal of the second transistor is coupled to a second differential mode signal line. The first and second differential mode signal lines provide the differential mode signal. In an embodiment, the signal conversion circuit also includes a first resistor and a second resistor. A first terminal of the first resistor is coupled to the tip signal, a second terminal of the first resistor is coupled to the common mode signal, a first terminal of the second resistor is coupled to the ring signal, and a second terminal of the second resistor is coupled to the common mode signal.

In an alternative embodiment, the subscriber line interface circuit apparatus also includes a first fuse device and a second fuse device. The first fuse device is disposed between the first node and the first signal line, whereas the second fuse device is disposed between the second node and the second signal line. In this embodiment, the signal conversion circuit includes a first transistor and a second transistor. The first transistor includes a base terminal coupled to the first node, an emitter terminal coupled to the ring signal, and a collector terminal coupled to a first differential mode signal line. The second transistor includes a base terminal coupled to the second node, an emitter terminal coupled to the tip signal, and a collector terminal coupled to a second differential mode signal line. In this signal conversion circuit, the first and second differential mode signal lines provide the differential mode signal.

In a specific embodiment of the subscriber line interface circuit apparatus described above, the tip control circuit includes a first and a second bipolar transistors. An emitter terminal of each of the first and second tip control bipolar transistors is in communication with the first tip control current signal and the second tip control current signal, respectively. In an embodiment, a base terminal of the first tip control transistor is in communication with the tip control voltage signal. In a specific embodiment, the ring control circuit includes a first and a second ring control bipolar transistors. An emitter terminal of each of the first and second ring control bipolar transistors is in communication with the first ring control current signal and the second ring control current signal, respectively. In an embodiment, a base terminal of the first ring control transistor is in communication with the ring control voltage signal. In a specific embodiment, the linefeed circuit comprises discrete components. In another embodiment, the linefeed circuit is implemented in an integrated circuit.

According to another embodiment of the invention, an integrated circuit apparatus is provided for controlling a subscriber loop. The integrated circuit apparatus includes a common mode input and a first and a second differential mode inputs. The common mode input receives a common mode signal, and the first and second differential mode inputs receive a differential mode signal. In a specific embodiment, the common mode input signal is related to a sum of the tip signal and the ring signal. In an embodiment, the differential mode signal is related to a difference between the tip signal and the ring signal. In an embodiment, each of the common mode signal and the differential mode signal is related to a tip signal and a ring signal of the subscriber loop. In a specific embodiment, each of the common mode signal and the differential mode signal is generated outside the integrated circuit apparatus. The integrated circuit apparatus is isolated from the tip signal and the ring signal. In other words, the integrated circuit apparatus does not receive a sensed tip signal or a sensed ring signal. The integrated circuit apparatus includes an analog to digital converter circuit which is coupled to the common mode input and the differential mode inputs. The integrated circuit apparatus includes an analog control circuit which is coupled to the common mode input and the differential mode inputs and provides individual tip and ring control signals in response to at least the common mode signal and the differential mode signal, without providing DC common mode control of the Tip and Ring signals. These control signals include the following:

-   -   1. A first and second tip control current signals provided by a         first and a second tip control outputs, respectively, which are         coupled to the analog control circuit;     -   2. A first and second ring control current signals provided by a         first and a second ring control outputs, respectively, which are         coupled to the analog control circuit;     -   3. A tip control voltage signal provided by a tip control         voltage output, which is coupled to the analog control circuit;         and     -   4. A ring control voltage signal provided by a ring control         voltage output, which is coupled to the analog control circuit.

In an embodiment, the integrated circuit apparatus also includes an amplifier circuit for receiving the voice or audio band signal, respectively, an impedance matching circuit couple to the amplifier, a hybrid circuit coupled to the impedance matching circuit, and a codec coupled to the hybrid circuit. In an embodiment, the analog control circuit further comprising an analog voltage feedback circuit, a current feedback circuit, and a summation circuit.

According to an alternative embodiment of the invention, a subscriber line interface circuit apparatus is provided for a subscriber loop. The apparatus includes a linefeed circuit and a subscriber line control circuit. In an embodiment, the linefeed circuit includes a first node coupled to a first signal line and a second node coupled to a second signal line to receive a tip signal and a ring signal, respectively, from the subscriber loop. The linefeed circuit includes a signal conversion circuit which is coupled to the first node and the second node to receive the tip signal and the ring signal, respectively. The signal conversion circuit provides a differential mode signal and a common mode signal in response to at least the tip signal and the ring signal. In a specific embodiment, the common mode input signal is associated with a sum of the tip signal and the ring signal, and the differential mode input signal is associated with a difference between the tip signal and the ring signal. The linefeed circuit includes a filter circuit which is coupled to the signal conversion circuit and provides a voice band or audio band signal. The linefeed circuit also includes a tip control circuit and a ring control circuit. The tip control circuit is coupled to the first node and provides a tip output signal to the first signal line in response to a first and a second tip control current signals. The ring control circuit is coupled to the second node and provides a ring output signal to the second signal line in response to a first and a second ring control current signals. In an embodiment, a subscriber line control circuit (SLCC) is provided in a single integrated circuit chip. In a specific embodiment, the SLCC is coupled to the linefeed circuit and is isolated from the first and second signal lines. In other words, the SLCC does not sense the tip signal or the ring signal. The SLCC includes a first and a second differential mode inputs and a common mode input. The differential mode inputs and common mode input receive the differential mode signal and the common mode signal, respectively, from the linefeed circuit.

In a specific embodiment, the SLCC includes an analog to digital converter circuit which is coupled to the common mode input and the differential mode inputs. The SLCC also includes an analog control circuit which is coupled to the common mode input and the differential mode inputs. The analog control circuit provides the individual tip and ring control signals in response to at least the common mode signal and the differential mode signal, without providing DC common mode control of the Tip and Ring signals. These control signals are described below. The SLCC includes a first and a second tip control outputs coupled to the analog control circuit and provides the first and second tip control current signals, respectively. The SLCC also includes a first and a second ring control outputs coupled to the analog control circuit and provides the first and second ring control current signals, respectively. The SLCC includes an amplifier circuit for receiving the voice band or audio band signal, respectively, from the linefeed circuit, an impedance matching circuit which is couple to the amplifier, a hybrid circuit coupled to the impedance matching circuit, and a codec which is coupled to the hybrid circuit. In a specific embodiment, the linefeed circuit includes discrete components, whereas in another embodiment, the linefeed circuit is implemented in an integrated circuit.

Many benefits are provided by way of the present invention over conventional techniques. In a specific embodiment, the invention provides a subscriber loop control circuit which includes differential and common-mode signals, and is relatively isolated from the subscriber loop. There is no need for a signal processor to calculate the common mode and differential mode signals. In an embodiment, the invention provides feedback common mode and differential mode input signals that allow remote detection of a subscriber line circuit faulty condition as indicated by a melted fuse. For example, by comparing the common mode voltage and differential mode voltages against target voltages, the subscriber line control circuit can determine whether the fuse between the subscriber loop and the linefeed circuit is melted. This remote fault detection capability can reduce the cost of a subscriber line service. In some embodiments, the invention provides a subscriber line control circuit that is relatively less expensive than conventional circuits. Embodiments of the invention have a broad range of applicability. For example, certain embodiments of the invention are useful in packet switched telephony applications, which use a ‘plain old telephone’ interface. In an embodiment, the invention can be used in residential VOIP. A specific embodiment of the invention can be used in a device called an ATA (analog terminal adapter), which has an interface to the ‘plain old telephone’. The above-mentioned remote fault detection capability can be especially advantageous in reducing the cost of service in residential ATA's. In another example, embodiments of the invention can be used with an Ethernet port to be in communication with an Internet router. These and other benefits will be described in more detail throughout the present specification and more particularly below.

Various additional objects, features, and advantages of the present invention can be more fully appreciated with reference to the detailed description and accompanying drawings that follow.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified block diagram of a subscriber line interface circuit (SLIC) 100 according to an embodiment of the present invention;

FIG. 2 a is a simplified block diagram of a subscriber line interface circuit (SLIC) 200 according to an another embodiment of the present invention; and

FIG. 2 b is a simplified block diagram of a subscriber line control circuit (SLCC) 210 according to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to integrated circuits. More particularly, the invention provides a method and device for an integrated circuit for telecommunication. Merely by way of example, the invention has been applied to subscriber line control circuits for interfacing with digital equipment in a central office. But it would be recognized that the invention has a much broader range of applicability. For example, embodiments of the invention can find application in communication networks such as cable television networks, fiber optic, Ethernet port interface to the Internet, VoIP, and wireless local loop, etc.

As discussed above, conventional subscriber line interface chipset combination of a SLIC chip and a CODEC chip have certain limitations. A disadvantage of this conventional design is that it involves a large number of circuit components on the high voltage and high power SLIC integrated circuit. As a result, conventional designs can be relatively expensive. Alternative designs have been suggested. For example, a technique for partitioning the SLIC into multiple integrated circuits is presented in K. Kawarada et al, “High Voltage Subscriber Line Interface LSIs”, IEEE Journal of Solid-State Circuit, Vol. SC-17, No. 6, pp. 1144-1149, December 1982. As shown in Kawarada et al FIG. 2, certain control functions of the SLIC are implemented in a CMOS chip (CONT). A high voltage bipolar LSI chip (RT) performs the ringing and testing functions. A medium voltage bipolar LSI chip (BSH) performs the battery feed, supervision, and hybrid functions. Kawarada et al also includes another CMOS chip for the CODEC function. Even though the techniques provided by Kawarada et al may have some benefits such as moving certain circuit components to a low voltage CMOS chip, there are many limitations. For example, a large number of separate integrated circuits are needed. In addition, signals on the tip and ring lines are directly fed into the RT and BSH circuits. They are less isolated from the tip and ring lines. As a result, the circuits may be susceptible to variations in the tip and ring lines.

In another example, in U.S. Pat. No. 6,934,384 by Hein et al, a subscriber loop interface circuit apparatus includes a signal processor having sense inputs for a sensed tip signal and a sensed ring signal of a subscriber loop. The signal processor generates a linefeed driver control signal in response to the sensed signals and the signal processor resides on a same integrated circuit die as a codec for bi-directional communication of voiceband data between the analog subscriber loop and a digital interface of the signal processor. In U.S. Pat. No. 6,934,384, a signal processor 210 can be found in FIG. 2 and sensed tip signals 332 and 334 and sensed ring signals 336 and 338 can be found in FIG. 3. U.S. Pat. No. 6,934,384 also provides an apparatus including a signal processor generating subscriber loop control signals in response to a sensed tip signal and a sensed ring signal of a subscriber loop, and a linefeed driver portion for driving the subscriber loop in accordance with the subscriber loop control signals, the linefeed driver portion providing the sensed tip and ring signals. Each of the linefeed driver portion and the signal processor resides on an integrated circuit die, wherein the signal processor resides on a same integrated circuit die as a codec for bi-directional communication of voiceband data between the analog subscriber loop and a digital interface of the signal processor. A signal processor 210 and a linefeed driver 220 can be found in FIG. 2 of Hein et al. Sensed tip signals 332 and 334 and sensed ring signals 336 and 338 can be found in FIG. 3. Bi-direction voiceband data lines 322 324 and 326 and 328 can be found in FIG. 3 of Hein et al. Even though the techniques provided by U.S. Pat. No. 6,934,384 may have some benefits, there are many limitations. For example, in U.S. Pat. No. 6,934,384 the signal processor senses the tip and ring lines, being less isolated from the tip and ring lines. As a result, the signal processor may be susceptible to variations in the tip and ring lines. Moreover, in U.S. Pat. No. 6,934,384, the common mode and differential mode components are calculated by the signal processor rather than the linefeed driver. A disadvantage of this design is that the signal processor requires additional circuitry and power consumption. This tends to increase the cost of the design and manufacture. Therefore, improved techniques for subscriber line interface circuit are desired.

According embodiments of the present invention, improved techniques for subscriber line interface circuit are provided. Depending upon the embodiment, the present invention includes various features, which may be used. These features include the following:

-   -   1. A subscriber line interface circuit apparatus for a         subscriber loop that includes a linefeed circuit in combination         with subscriber line control circuitry (SLCC);     -   2. A linefeed circuit that includes a unique signal conversion         circuit which provides both a differential mode signal and a         related common mode signal in response to at least the tip         signal and the ring signal so the SLCC circuitry need not derive         such signals;     -   3. A subscriber line control circuit (SLCC) which receives the         common mode and differential mode signals. The common mode and         differential mode signals can be used directly for monitoring         the common mode and differential mode voltages on the subscriber         line;     -   4. A subscriber line control circuit (SLCC) that is isolated         from the tip signal and the ring signal by the linefeed circuit.         The SLCC is therefore relatively independent of the         instabilities in the tip signal and the ring signal; and     -   5. A subscriber line interface circuit apparatus in which a         linefeed circuit provides common mode and differential mode         signals which can be used for remote detection of a faulty         condition on the subscriber line.

As shown, the above features may be in one or more of the embodiments to follow. These features are merely examples, which should not unduly limit the scope of the claims herein. One of ordinary skill in the art would recognize many variations, modifications, and alternatives. It is also noted that in this application, the term “subscriber line” and the term “subscriber loop” are used interchangeably.

FIG. 1 is a simplified block diagram of a subscriber line interface circuit (SLIC) 100 according to an embodiment of the present invention. This diagram is merely an example, which should not unduly limit the scope of the claims herein. One of ordinary skill in the art would recognize other variations, modifications, and alternatives. As shown, SLIC circuit 100 includes a subscriber loop control circuit (SLCC) 110 and a linefeed circuit 120. In an embodiment, the subscriber loop control circuit 110 provides control signals 126 to the linefeed circuit in response to common mode and/or differential subscriber loop signals 122 from the linefeed circuit. In a specific embodiment, the common mode and/or differential mode signals are provided by the linefeed circuit 120 and do not need to be calculated by the subscriber loop control circuit 110. Therefore, they can be used directly in the subscriber loop control circuit 110 for monitoring the common mode and differential mode voltages on the subscriber line. In an embodiment, a common mode subscriber loop signal can be either a voltage or a current. For example, a common mode subscriber loop signal can be related to a sum of the subscriber loop voltages. In a specific embodiment, a common mode subscriber loop signal is proportional to a sum of the subscriber loop voltages, e.g. Tip signal 133 and Ring signal 135. In an embodiment, the differential subscriber loop signals can either be a voltage or a current. In another example, the differential subscriber loop signal can be related to the subscriber loop signals, e.g. Tip signal 133 and Ring signal 135. In a specific embodiment, the differential subscriber loop signal can be proportional to the voltage across the subscriber loop.

According to an embodiment of the invention, a subscriber line interface circuit (SLIC) is provided that includes a subscriber loop control circuit (SLCC) and a linefeed circuit. In an embodiment, the SLCC is provided in a single integrated circuit chip. In a specific embodiment, the SLCC and the linefeed circuit can be built within a single integrated circuit package. In another embodiment, the SLCC and the linefeed circuit can be in separate IC packages. In an alternative embodiment, the linefeed circuit is implemented using discrete components. In an embodiment, the interface between a linefeed circuit and a subscriber loop control circuit can include a common mode and/or a differential mode subscriber loop signal and voice band or audio band data signals. In some embodiments, audio band includes a frequency range of about 20 Hz to 20,000 Hz and voice band has a frequency range of about 300 Hz to 3,400 Hz.

Referring to FIG. 1, in a specific embodiment of the invention, subscriber loop control circuit 110 receives common mode and differential mode signals 122 and voice band and/or audio band signals 124 from the linefeed circuit 120. Subscriber loop control circuit 110 generates tip/ring control signals 126 which include superimposed audio/voice band signals, in response to at least the common mode/differential mode signals, the voice band and/or audio band signals, the control interface 105 and the Voice band and/or audio band interface 107. In an embodiment, the control interface 105 and the Voice band and/or audio band interface 107 are coupled to a digital communication network. Merely as examples, the digital communication network can be found in a central office of a telephone service provider, cable television networks, fiber optic, Ethernet port interface to the Internet, VoIP, and wireless local loop, etc. In a specific embodiment, the invention provides a subscriber loop control circuit which includes a programmable CODEC, a programmable hybrid and programmable subscriber line impedance matching for worldwide deployment. In another embodiment, the invention provides a combination of a subscriber loop control circuit and linefeed circuit, which provides substantially all the BORSCHT functions required for subscriber line operation. Of course, there can be other variations, modifications, and alternatives.

FIG. 2 a is a simplified block diagram of a subscriber line interface circuit (SLIC) 200 according to another embodiment of the present invention. This diagram is merely an example, which should not unduly limit the scope of the claims herein. One of ordinary skill in the art would recognize other variations, modifications, and alternatives. As shown, SLIC circuit 200 includes a subscriber loop control circuit (SLCC) 210 and a linefeed circuit 220. The linefeed circuit 220 includes TIP control circuit 221, RING control circuit 222, a signal conversion circuit 223, and a filter circuit 224. In an embodiment, linefeed circuit 220 includes circuit nodes N_(TIP) and N_(RING). Node N_(TIP) is coupled to a signal line to receive a tip signal TIP from the subscriber loop. Node N_(RING) is coupled to a second signal line to receive a ring signal RING from the subscriber loop. In a specific embodiment, the linefeed circuit 220 also includes a fuse FT coupled between node N_(TIP) and tip signal TIP from the subscriber loop, and a second fuse FR and coupled between node N_(RING) and ring signal RING from the subscriber loop. In an alternative embodiment, fuses FT and FR can be omitted.

As shown in FIG. 2 a, signal conversion circuit 223 provides a common mode signal S_(COM) and a differential mode signal S_(DIFF) in response to the tip signal TIP and the ring signal RING of the subscriber loop. In a specific embodiment, the signal conversion circuit 223 includes a resistor R13 coupled between node N_(TIP) and common mode signal S_(COM), and a second resistor R14 between node N_(RING) and common mode signal S_(COM). In a specific embodiment, the common-mode signal S_(COM) is proportional to a sum of the subscriber loop tip and ring voltages. In an embodiment, signal conversion circuit 223 includes transistors QD1 and QD2, and resistors RD1, RD2, RD3, and RD4 as shown in FIG. 2 a. In an embodiment, the differential mode signal is provided across a first differential signal line SD1 and a second differential signal line SD2 as shown in FIG. 2 a. In a specific example, transistors QD1 and QD2 are bipolar transistors. As shown, resistor RD4 is coupled between a base terminal of QD1 and node N_(TIP), and resistor RD3 is coupled between a base terminal of QD2 and node N_(RING). In a specific embodiment, resistor RD2 is coupled between an emitter terminal of QD1 and the ring signal RING from the subscriber loop, and resistor RD1 is coupled between an emitter terminal of QD2 and the tip signal TIP from the subscriber loop. In the embodiment shown in FIG. 2 a, differential mode signal S_(DIFF) is provided across signal lines SD1 and SD2, which are coupled to a collector terminal of transistor QD1 and a collector terminal of transistor QD2, respectively. In an embodiment, differential mode signal S_(DIFF) is proportional to a difference between the tip signal and the ring signal. Of course, there can be other variations, modifications, and alternatives. For example, in an alternative embodiment, the emitter terminal of QD1 and the base terminal of QD2 are both coupled to node N_(RING), and an emitter terminal of QD2 and a base terminal of QD1 are both coupled to node N_(TIP).

As shown in FIG. 2 a, linefeed circuit 220 includes a filter circuit 224, which includes capacitor CRA and resistor R6 connected in series to provide an audio band signal input terminal RAC of SLCC 210. Similarly, filter circuit 224 includes capacitor CTA and resistor R4 connected in series to provide an audio band signal to input terminal TAC of SLCC 210. In some embodiments of the present invention, audio band includes a frequency range of about 20 Hz to 20,000 Hz, and voice band has a frequency range of about 300 Hz to 3,400 Hz.

FIG. 2 b is a simplified block diagram of a subscriber line control circuit (SLCC) 210 according to an embodiment of the present invention. This diagram is merely an example, which should not unduly limit the scope of the claims herein. One of ordinary skill in the art would recognize other variations, modifications, and alternatives. As shown, the subscriber line control circuit 210 includes amplifier circuit 209, impedance matching circuit 211, hybrid circuit block 212, and programmable codec 213. In an embodiment, SLCC 210 also includes analog control circuit 208, digital-to-analog converter circuit (DAC) 217, analog-to-digital circuit (ADC) 218, and logic circuit 219. In a specific embodiment, analog control circuit 208 includes a summation circuit 214, analog voltage feedback circuit 215, and current feedback circuit 216. In an embodiment, the subscriber line control circuit 210 includes input terminals RAC and TAC for receiving voice band or audio band signals from the linefeed circuit 220. In some embodiments, these circuit blocks together implement many of the BORSCHT functions. For example the amplifier 209 receives AC voice band or audio band signals from the linefeed circuit 220. The impedance matching circuit 211 matches the characteristic impedance of the subscriber line. In an embodiment, the hybrid circuit 212 performs 2-wire to 4-wire conversion. The codec 213 performs coding and decoding functions for the voice or audio band signals. In a preferred embodiment, the SLCC is provided in a single integrated circuit chip. Of course, there can be other variations, modifications, and alternatives.

In a specific embodiment, SLCC 210 includes an input SCM for receiving the common mode signal S_(COM) from the linefeed circuit, and also includes inputs SD1 and SD2 for receiving the differential mode signal S_(DIFF) from the linefeed circuit. As shown in FIGS. 2 a and 2 b, the SLCC 210 is isolated from the tip signal TIP and the ring signal RING by the signal conversion circuit 223 in the linefeed circuit. In other words, SLCC 210 does not sense the tip signal or the ring signal according to embodiments of the present invention. In contrast, SLCC 210 receives a differential mode signal and a common mode signal, and can be relatively independent from the instabilities of the tip and ring signals.

As shown in FIG. 2 b, in an embodiment, SLCC 210 includes output terminals TPP and TPN for providing tip control current signals. Similarly, SLCC 210 includes output terminals RIP and RIN for providing ring control current signals. In a specific embodiment, the subscriber line control circuit 210 also includes terminals VBT and VBR for providing tip and ring control voltage signals, respectively. These tip and ring control current and voltage signals are provided by the analog control circuit 208 in response to at least the common mode signal S_(COM) and differential mode signal S_(DIFF) from the linefeed circuit 220, and signals derived from the control interface 205 and voice/audio band interface 207. In an embodiment, the control interface 205 and the Voice band and/or audio band interface 207 are coupled to a digital communication network. For example, the digital communication network can be found in a central office of a telephone service provider, cable television networks, fiber optic, Ethernet port interface to the Internet, VoIP, or wireless local loop, etc. Of course, there can be other variations, modifications, and alternatives.

Referring back to FIG. 2 a, in a specific embodiment, the tip control signals TPP and TPN are received by tip control circuit 221, which is coupled to the N_(TIP) node for controlling the TIP signal line. The tip control circuit 221 includes transistors QT1, QT2, and QT3, capacitor CT1, and resistors RT1, RT2, and RT3. In an embodiment, the tip control signals TPP and TPN are received at an emitter terminal of transistors QT1 and QT2, respectively. The tip control circuit 221 is also coupled to capacitors CTL for line capacitance compensation. Resistor R11 is coupled between output terminal TVE and an emitter of QT3 in the tip control circuit 221. In an embodiment, the subscriber line control circuit 210 includes output terminals RIP and RIN for providing ring control signals. In a specific embodiment, the ring control signals RIP and RIN are received by ring control circuit 222, which is coupled to the N_(RING) node for controlling the RING signal line. In an embodiment, ring control circuit 222 includes transistors QR1, QR2, and QR3, capacitor CR1, and resistors RR1, RR2 and RR3. In an embodiment, the ring control signals RIP and RIN are received at an emitter terminal of transistors QR1 and QR2, respectively. The ring control circuit 222 is also coupled to capacitors CRL for line capacitance compensation. Resistor R12 is coupled between output terminal RVE and an emitter of QR3 in the ring control circuit 222. In an embodiment, SLCC 210 includes a terminal BAT for communication with a VBAT signal through resistor R10. In an embodiment, VBAT is a supply voltage for the high voltage linefeed circuit. In an example, VBAT can be provided by an external power source. In another example, the SLCC includes an on-chip DCDC controller which can be used to control VBAT. As shown in FIG. 2 a, VBAT is connected to resistors RR2, RR3, RT2, RT3, and RIO, and capacitors CR1 and CT1. Of course, there can be other variations, modifications, and alternatives.

As discussed above, in a specific embodiment, the subscriber line control circuit 210 also includes terminals VBT and VBR, which provide voltage signals to tip control circuit 221 and ring control circuit 222, respectively. As shown in FIG. 2 a, a base terminal of linefeed transistors QT1 receives a voltage control signal from terminal VBT of the SLCC 210. Similarly, a base terminal of linefeed transistors QR1 receives a second voltage control signal from terminal VBR of the SLCC 210. In an embodiment, the currents through transistors QR1 and QT1 are proportional to the voltage across resistors RRE or RTE, respectively. In a specific embodiment, resistors RRE or RTE are coupled to a reference voltage VREF. In a specific embodiment, VREF is an external reference voltage. In such embodiment, the currents through transistors QR1 and QT1 are supplied predominantly by the external reference voltage VREF, whereas voltage signals VBR and VBT from SLCC 210 are used to control the current through transistors QR1 and QT1, respectively. This specific embodiment offers many advantages. For example, the SLCC does not need to supply large currents to the linefeed circuit. In conventional design, these current may be in the order of 20-80 mA. According to an embodiment of the present invention, current consumption of the integrated circuit is reduced and large integrated driver circuits can be avoided. As a result, integrated circuit heat sinks are not needed. In an example, VREF can be derived from a power supply voltage. In a particular example, VREF is equal to the supply voltage in magnitude. Of course, there can be other variations, modifications, and alternatives.

As discussed above, in a specific embodiment, fuses FT and FR are provided in the linefeed circuit 220 as shown in FIG. 2 a. A faulty condition in the subscriber loop can cause either or both fuses to be blown. In an embodiment, the present invention provides a method for remote detection of the faulty condition using the signal conversion circuit 223 As shown in FIG. 2 a, nodes N_(TIP) and N_(RING) receive tip and ring control signals from the tip control circuit 221 and ring control circuit 222, respectively. Under a typical operating condition, the signals at nodes N_(TIP) and N_(RING) are coupled to the TIP and RING signal lines of the subscriber loop. If either or both of the fuses FT and FR are blown, the TIP or RING lines of the subscriber loop can not receive these signals. In a specific embodiment, the signal conversion circuit 223 provides common-mode and differential mode feedback signals S_(COM) and S_(DIFF) which can be used to determine whether either or both fuses FT and FR are blown. In this embodiment, resistor RD4 is coupled between a base terminal of transistor QD1 and node N_(TIP), and resistor RD2 is coupled between an emitter terminal of QD1 and the RING signal line on the outside of fuse FR. Similarly, resistor RD3 is coupled between a base terminal of transistor QD2 and node N_(RING), and resistor RD1 is coupled between an emitter terminal of QD2 and the TIP signal line on the outside of fuse FT. Under a faulty condition, for example, if fuse FT is blown, the emitter terminal of transistor QD2, coupled to the blown fuse FT through resistor RD1, does not receive the expected signal at node N_(TIP), causing an erroneous differential mode signal S_(DIFF). On the other hand, the expected common-mode signal S_(COM) associated with the signals on nodes N_(TIP) and N_(RING) is still available through resistors R13 and R14. By examining the common-mode and differential mode signals, the SLCC 210 can determine that the TIP line of the subscriber loop has a faulty condition. Similarly, a faulty condition in the RING line of the subscriber loop can also be detected.

According to embodiments of the present invention, subscriber line interface circuit (SLIC) 200 can be implemented in various circuit configurations. For example, on a specific embodiment, subscriber line control circuit (SLCC) 210 is implemented in an integrated circuit, and linefeed circuit 220 may be implemented using discrete components. In another embodiment, the SLCC is implemented in an integrated circuit and the linefeed circuit is implemented in a second integrated circuit. For example, the SLCC can be implemented in a CMOS integrated circuit. In another example, the linefeed circuit can be implemented in a bipolar integrated circuit. In an alternative embodiment, the SLCC and the linefeed circuit are implemented in a single integrated circuit, e.g. a BiCMOS integrated circuit. Of course, there can be other variations, modifications, and alternatives.

Although the above has been shown using a selected group of components for the microphone interface circuit, there can be many alternatives, modifications, and variations. For example, some of the components may be expanded and/or combined. Other components may be inserted to those noted above. Depending upon the embodiment, the arrangement of components may be interchanged with others replaced. Further details of these components are found throughout the present specification and the drawings.

It is also understood that the examples and embodiments described herein are for illustrative purposes only and that various modifications or changes in light thereof will be suggested to persons skilled in the art and are to be included within the spirit and purview of this application and scope of the appended claims. 

1. A subscriber line interface circuit apparatus for a subscriber loop, the apparatus comprising: a linefeed circuit, the linefeed circuit including: a first node, the first node being coupled to a first signal line to receive a tip signal from the subscriber loop; a second node, the second node being coupled to a second signal line to receive a ring signal from the subscriber loop; a signal conversion circuit, the signal conversion circuit being coupled to the first node and the second node to receive the tip signal and the ring signal, respectively, the signal conversion circuit providing a differential mode signal and a common mode signal in response to at least the tip signal and the ring signal; a filter circuit, the filter circuit being coupled to the signal conversion circuit, the filter circuit providing audio band or voice band signals; a tip control circuit, the tip control circuit being coupled to the first node, the tip control circuit providing a tip output signal to the first signal line in response to a first tip control current signal, a second tip control current signal and a tip control voltage signal; and a ring control circuit coupled to the second node, the ring control circuit providing a ring output signal to the second signal line in response to a first ring control current signal, a second ring control current signal, and a ring control voltage signal; and a subscriber line control circuit, the subscriber line control circuit being provided in a single integrated circuit chip, the subscriber line control circuit being coupled to the linefeed circuit, the subscriber line control circuit being isolated from the first and second signal lines, the subscriber line control circuit including: a first and a second differential mode inputs and a common mode input, the differential mode inputs receiving the differential mode signal from the linefeed circuit, the common mode input receiving the common mode signal from the linefeed circuit; an analog to digital converter circuit, the analog to digital converter circuit being coupled to the common mode input and the differential mode inputs; an analog control circuit, the analog control circuit being coupled to the common mode input and the differential mode inputs, the analog control circuit providing the first and second tip control current signals, the tip control voltage signal, the first and second ring control current signals, and the ring control voltage signal, in response to at least the common mode signal and the differential mode signal; a first and a second tip control outputs coupled to the analog control circuit, the first and second tip control outputs providing the first and second tip control current signals, respectively; a first and a second ring control outputs coupled to the analog control circuit, the first and second ring control outputs providing the first and second ring control current signals, respectively; a tip control voltage output coupled to the analog control circuit, the tip control voltage output providing the tip control voltage signal to the linefeed circuit; and a ring control voltage output coupled to the analog control circuit, the ring control voltage output providing the ring control voltage signal to the linefeed circuit; an amplifier circuit for receiving audio band or voice band signals from the linefeed circuit; an impedance matching circuit, the impedance matching circuit being coupled to the amplifier circuit, the impedance matching circuit being configured for the subscriber line control circuit to match a characteristic line impedance of the subscriber loop; a hybrid circuit, the hybrid circuit being coupled to the impedance matching circuit; and a codec, the codec being coupled to the hybrid circuit.
 2. The subscriber line interface circuit apparatus of claim 1 wherein the common mode input signal is associated with a sum of the tip signal and the ring signal.
 3. The subscriber line interface circuit apparatus of claim 1 wherein the differential mode signal is associated with a difference between the tip signal and the ring signal.
 4. The subscriber line interface circuit apparatus of claim 1 wherein the linefeed circuit isolates the subscriber line control circuit from the first and second signal lines, and wherein the linefeed circuit prevents the subscriber line control circuit from sensing the tip signal and the ring signal.
 5. The subscriber line interface circuit apparatus of claim 1, wherein the signal conversion circuit comprises a first transistor and a second transistor, wherein a first terminal of the first transistor is coupled to the tip signal, a second terminal of the first transistor is coupled to the ring signal, a third terminal of the first transistor is coupled to a first differential mode signal line, a first terminal of the second transistor is coupled to the ring signal, a second terminal second transistor is coupled to the tip signal, and a third terminal of the second transistor is coupled to a second differential mode signal line, the first and second differential mode signal lines providing the differential mode signal.
 6. The subscriber line interface circuit apparatus of claim 5 where in the signal conversion circuit further comprises a first and second resistors, wherein a first terminal of the first resistor is coupled to the tip signal, a second terminal of the first resistor is coupled to the common mode signal, a first terminal of the second resistor is coupled to the ring signal, and a second terminal of the second resistor is coupled to the common mode signal.
 7. The subscriber line interface circuit apparatus of claim 1, further comprising a first fuse device and a second fuse device, the first fuse device being disposed between the first node and the first signal line, the second fuse device being disposed between the second node and the second signal line.
 8. The subscriber line interface circuit apparatus of claim 7, wherein the signal conversion circuit comprises: a first transistor, the first transistor including a base terminal coupled to the first node, an emitter terminal coupled to the ring signal, and a collector terminal coupled to a first differential mode signal line; and a second transistor, the second transistor including a base terminal coupled to the second node, an emitter terminal coupled to the tip signal, and a collector terminal coupled to a second differential mode signal line; wherein the first and second differential mode signal lines provide the differential mode signal.
 9. The subscriber line interface circuit apparatus of claim 1, wherein the tip control circuit comprises a first and a second bipolar transistors, an emitter terminal of each of the first and second tip control bipolar transistors being in communication with the first tip control current signal and the second tip control current signal, respectively, and a base terminal of the first tip control transistor being in communication with the tip control voltage signal.
 10. The subscriber line interface circuit apparatus of claim 1, wherein the ring control circuit includes a first and a second ring control bipolar transistors, an emitter terminal of each of the first and second ring control bipolar transistors being in communication with the first ring control current signal and the second ring control current signal, respectively and a base terminal of the first ring control transistor being in communication with the ring control voltage signal.
 11. The subscriber line interface circuit apparatus of claim 1 wherein the linefeed circuit comprises discrete components.
 12. The subscriber line interface circuit apparatus of claim 1 wherein the linefeed circuit is implemented in an integrated circuit.
 13. An integrated circuit apparatus for controlling a subscriber loop, the integrated circuit apparatus comprising: a common mode input and a first and a second differential mode inputs, the common mode input receiving a common mode signal, the first and second differential mode inputs receiving a differential mode signal, respectively, each of the common mode signal and the differential mode signal being related to a tip signal and a ring signal of the subscriber loop, each of the common mode signal and the differential mode signal being generated outside the integrated circuit apparatus, the integrated circuit apparatus being isolated from the tip signal and the ring signal; an analog to digital converter circuit, the analog to digital converter circuit being coupled to the common mode input and the first and second differential mode inputs; a first and a second tip control outputs coupled to the analog control circuit, the first and second tip control outputs providing the first and second tip control current signals, respectively; a first and a second ring control outputs coupled to the analog control circuit, the first and second ring control outputs providing the first and second ring control current signals, respectively; a tip control voltage output coupled to the analog control circuit, the tip control voltage output providing the tip control voltage signal; and a ring control voltage output coupled to the analog control circuit, the ring control voltage output providing the ring control voltage signal; an amplifier circuit for receiving audio band or voice band signals. an impedance matching circuit, the impedance matching circuit being coupled to the amplifier circuit, the impedance matching circuit being configured to match a characteristic line impedance of the subscriber loop; a hybrid circuit, the hybrid circuit being coupled to the impedance matching circuit; and a codec, the codec being coupled to the hybrid circuit.
 14. The integrated circuit apparatus of claim 13 wherein the common mode input signal is related to a sum of the tip signal and the ring signal.
 15. The subscriber line interface circuit apparatus of claim 13 wherein the differential mode input signal is related to a difference between the tip signal and the ring signal.
 16. A subscriber line interface circuit apparatus for a subscriber loop, the apparatus comprising: a linefeed circuit, the linefeed circuit including: a first node, the first node being coupled to a first signal line to receive a tip signal from the subscriber loop; a second node, the second node being coupled to a second signal line to receive a ring signal from the subscriber loop; a signal conversion circuit, the signal conversion circuit being coupled to the first node and the second node to receive the tip signal and the ring signal, respectively, the signal conversion circuit providing a differential mode signal and a common mode signal in response to at least the tip signal and the ring signal; a filter circuit, the filter circuit being coupled to the signal conversion circuit, the filter circuit providing audio band or voice band signals; a tip control circuit, the tip control circuit being coupled to the first node, the tip control circuit providing a tip output signal to the first signal line in response to a first and a second tip control current signals; and a ring control circuit coupled to the second node, the ring control circuit providing a ring output signal to the second signal line in response to a first and a second ring control current signals; and a subscriber line control circuit, the subscriber line control circuit being provided in a single integrated circuit chip, the subscriber line control circuit being coupled to the linefeed circuit, the subscriber line control circuit being isolated from the first and second signal lines, the subscriber line control circuit including: a first and a second differential mode inputs, the first and second differential mode inputs receiving the differential mode signal from the linefeed circuit; a common mode input, the common mode input receiving the common mode signal from the linefeed circuit; an analog to digital converter circuit; the analog to digital converter circuit being coupled to the common mode input and the differential mode inputs; an analog control circuit, the analog control circuit being coupled to the common mode input and the differential mode inputs, the analog control circuit providing the first and the second tip control current signals, the tip control voltage signal, the first and second ring control current signals, and the ring control voltage signal, in response to at least the common mode signal and the differential mode signal; a first and a second tip control outputs coupled to the analog control circuit, the first and second tip control outputs providing the first and second tip control current signals, respectively; a first and a second ring control outputs coupled to the analog control circuit, the first and second ring control outputs providing the first and second ring control current signals, respectively; an amplifier circuit for receiving the audio band or voice band signals from the linefeed circuit; an impedance matching circuit, the impedance matching circuit being coupled to the amplifier circuit, the impedance matching circuit being configured for the subscriber line control circuit to match a characteristic line impedance of the subscriber loop; a hybrid circuit, the hybrid circuit being coupled to the impedance matching circuit; and a codec, the codec being coupled to the hybrid circuit.
 17. The subscriber line interface circuit apparatus of claim 16 wherein the common mode signal is associated with a sum of the tip signal and the ring signal.
 18. The subscriber line interface circuit apparatus of claim 16 wherein the differential mode signal is associated with a difference between the tip signal and the ring signal.
 19. The subscriber line interface circuit apparatus of claim 16 wherein the linefeed circuit comprises discrete components.
 20. The subscriber line interface circuit apparatus of claim 16 wherein the linefeed circuit is implemented in an integrated circuit. 